In radiotherapy, a CT (computed tomography) image in the affected area is first imaged beforehand at the planning stage, and a treatment plan is established. At the treatment stage, the affected area is treated by an affected area being radiated by a treatment beam. The affected area might move because of respiration or heartbeat, intestinal movement, or the like. Known treatment methods of handling this are the gated irradiation method and the tracking irradiation method. These irradiation methods enable a reduction of the amount of the treatment beam that strikes normal locations other than the affected area.
There are cases in which CT images of various respiratory phases are imaged immediately before treatment apart from use for treatment planning. In these cases, a CT image that is similar to CT image for treatment planning is selected from the CT images for the various respiratory phases. Irradiation is done automatically with the treatment beam when there is nearly coincidence of image information in the vicinity of thoracic diaphragm between the perspective image used for generating the selected CT image and the perspective image imaged during treatment.
With conventional techniques for automation, however, at the treatment stage or rehearsal stage, because treatment support is not provided by tracking the position of the patient, there have been cases in which the reliability was insufficient.